Is there a way to make a given value relative to another piece of value?
You can do that easily with drivers. Drivers can be hard to understand, but are very powerful.


RMB  click on the Offset field in the Follow Path constraint that you want to be linked to the other constraint's value (I will call this object 1). 
Chose Add driver
Open up the Graph Editor and switch it to Drivers, click on the Offset Factor (Follow Path) in the list of drivers on the left.
Press N to open up the Properties panel to edit the driver. 
Change the var type to Single Property, point the Prop: to the object that has the other Follow Path constraint.
Select the object that has the other Follow Path constraint (in my example it is Suzanne).
RMB  click on the Offset field in the Follow Path constraint chose Copy Data Path
Select object 1, go back to the Graph Editor, and paste in the value for the Path:, should be this constraints["Follow Path"].offset_factor.
Change the Expr: to be var + 10.